1. Field of the Invention
The present invention relates to the technical field of image processing and, more particularly, to a motion vector estimator for reducing film judder.
2. Description of Related Art
In shooting a film, movie signals are progressively scanned to thus produce progressively scanned frames. In order to transmit the movie signals in a television system (e.g., an NTSC or PAL system), the progressively scanned frame has to be divided into multiple fields. For displaying on a television, a de-interlacing scanner combines multiple fields belonging to the same frame and accordingly restores the progressively scanned frame.
However, the rate is 60 Hz for the shot fields of a television program, and it is 24 Hz for the shot frames of a film, which is lower than 60 Hz and thus causes serious film judder. For reducing the film judder caused by the lower frame rate, a motion estimation is used to find the moving speed of an object, and subsequently a motion compensation is used for an original frame to generate other time points based on the motion vectors and the tandem temporal frame data interpolation to thereby increase the final frame rate.
A motion estimation applied to such a frame rate conversion needs to find the true motion vectors of an object, not find the similar image portions only. The true motion vectors commonly have the highly spatial and temporal correspondence. U.S. Pat. No. 5,221,926 granted to Jackson for a “Circuit and method for cancelling nonlinearity error associated with component value mismatches in a data converter” is disclosed to add the known motion vectors found around a block and the different update vectors to generate a plurality of candidate vectors, perform a block matching on the candidate vectors to obtain the matching costs of each candidate vectors, and select a candidate vector with the smallest matching cost as the optimal vector for the block. Each block has a limited number of candidate vectors and a limited number of update vectors, but the recursive processing can be converged to the optimal motion vector.
An approach is proposed to select the spatial and temporal candidate vectors and perform a spatially and temporally recursive processing on the selected candidate vectors to thereby speed the convergence to the time of the true motion vector (see “True-Motion estimation with 3-D recursive search block matching”, IEEE transactions on circuits and systems for video technology, Vol. 3, No. 5, October 1993). In addition, the block matching calculation is performed only on a plurality of candidate vectors in order to relatively reduce the computational amount required in the motion estimation.
As cited, the spatially and temporally recursive block matching can relatively reduce the computational amount in the motion estimation and maintain the spatial and temporal correspondence of each motion vector to thereby converge to the appropriate speed of the motion vector and further determine the quality of the recursive motion estimation. The speed of a motion vector convergence is determined by three factors: one being whether the appropriate motion vectors are included in the candidate vectors, another being the magnitudes of the update vectors, and the other being the penalty cost for determining each candidate vector. The penalty cost indicates the preference for different candidate vector origination. The recursive operation can be converged to the appropriate motion vector more quickly by means of such a preference. Since an operational processing commonly applied to a television is limited by the left-to-right, up-to-down scanning on a TV frame, the candidate vectors can originate from the left or upper portions where the block matching is complete or from the previously temporal motion vectors. When either the spatial or the temporal motion vectors that are currently selected are not appropriate, it is possible that the pixels are located at the boundaries of different-rate objects. Thus, updating the present motion vectors is required for obtaining the appropriate motion vectors, and the magnitudes of updated vectors can determine the changing speed to the appropriate motion vectors. The speed is increased with an increase on the magnitude. However, the increased magnitudes of updated vectors has a poor resolution, resulting in the problem of poor convergence precision or vector oscillation when the changed motion vector between the object images is small.
Such a motion estimation technology is essentially used to overcome the aforementioned film judder. As cited, the movie signals are shot by progressive scanning. When a conversion to an interlaced scan is required in transmission, a complete (simultaneous shooting) progressively scanned frame is divided into a plurality of odd and even fields to transmit. For example, a frame is divided into one odd field and one even field, which is referred to as a 2:2 pull down. FIG. 1 is a schematic diagram of a typical 2:2 pull down. As shown in FIG. 1, the original movie signals such as F0, F2, F4 are converted into the odd and even fields E0, O0, E2, O2, E4, O4 in transmission, and the results F0, F1, F2, F3, F4 are outputted after motion estimation and compensation, where F0 is obtained by combining E0 and O0, F1 is the result of motion estimation and compensation of E0, O0, E2, O2, and so on.
FIG. 2 is a block diagram of a typical device for reducing a film judder. As shown in FIG. 2, the input video signal has a movie format of 2:2 pull down. After passing through three field buffers 210, 220, 230, the fields E2, O2 and fields E0, O0 are inputted to the motion estimator 240 and the motion compensator 250, respectively. The motion estimator 240 outputs the motion vector MV, which is subsequently fed back to the motion estimator 240 through the delay 260 to accordingly provide a source of future candidate vectors in motion estimation. Thus, a recursive search motion estimation configuration is established via such a feedback path.
FIG. 3 is a block diagram of a typical recursive motion estimator 240. In FIG. 3, the motion estimator 240 includes a motion vector selector 310, multiple block matchmakers 320 and a motion vector determinator 330. The motion vector selector 310 selects a plurality of candidate vectors from the vector sources, each candidate vector is based on its source to generate a penalty cost so that different sources of candidate vectors can indicate different matching preferences, and a high penalty cost indicates a low preference for the candidate vector. The block matchmakers 320 performs block matching on the selected candidate vectors, calculates the difference between two successive time points of each candidate vector, and adds the difference and the penalty cost of each candidate vector to thereby obtain the final matching costs of the candidate vectors. The motion vector determinator 330 selects a candidate vector with the lowest matching cost as a result of the motion estimation, i.e., a signal MV. The signal MV outputted by the motion vector determinator 330 passes through the delay 260 and enters into the motion vector selector 310 for a reference during a candidate vector selection.
However, the candidate vector selection and the updated magnitude of the motion vector in such a recursive process frequently influence the convergence speed and precision. The prior art does not consider how to speed up the convergence on the basis of ensuring the convergence precision. Therefore, it is desirable to provide an improved motion vector estimator to mitigate and/or obviate the aforementioned problems.